Fluorescent lamps are very popular light sources. A fluorescent lamp is a gas discharge tube. Generally, an inner surface of the wall of the tube is coated with light-emitting materials. Such light-emitting materials are usually fluorescent or phosphorescent metallic salts. The tube is filled with mercury vapor at extremely low pressure, and filaments are provided at each end of the tube. The light of the fluorescent lamp is not produced by an incandescent body (such as the filament of an ordinary electric lamp), but is emitted as a result of the excitation of atoms (namely, those of the mercury vapor and the fluorescent coating). Detailedly, electrons ejected from the cathode filaments collide with the mercury atoms of the vapor, and cause the mercury atoms to emit radiation. The radiation is mostly ultraviolet rays, which are invisible. The ultraviolet light strikes the fluorescent materials on the inner surface of the wall of the tube. Typically, this causes the fluorescent materials to emit radiation with a longer wavelength in the visible range of the spectrum. In this way, the coating transform the invisible ultraviolet rays into visible light.
A fluorescent lamp has certain advantages. Most notably, operation of the fluorescent lamp is highly economical compared to other light sources such as electric lamps. However, the fluorescent lamp also has certain drawbacks. For example, ultraviolet light needs to be transformed into visible light. Thus a certain amount of loss of light energy is inevitable. Further, there is a delay between powering on of the fluorescent lamp and the time when it begins to provide steady illumination. Additionally, relatively complicated control equipment is needed, which requires extra space. Moreover, some materials used in the fluorescent lamp, particularly mercury vapor, are liable to pollute the environment.
What is needed, therefore, is a clean light source with high light emission efficiency.